This invention relates to a system and method for extracting information from a carrier wave and relates generally to the method and apparatus described in U.S. Pat. Nos. 4,106,007 and 4,218,655, the disclosures of Which are incorporated herein by reference. As described in those patents, it is known that a modulation signal can be superimposed on a power system waveform, at specified locations on the power system waveform such as a zero crossing, to cause wave shape perturbations in the carrier wave. The embodiment described herein uses electric load pulses which are superimposed on the 60 Hz load current of an electrical power distribution system.
Communication over electric power distribution lines is useful for signaling, meter reading, and load control, among other uses. However, communication over an electric distribution system is a complex undertaking. Each customer service constitutes a branch in the distribution feeder, and the branching is so extensive that it is impractical to provide filter and by-pass circuitry at each branch point. The distribution system is not an attractive medium for conventional communications due to the attenuation and dispersion of the signals and because noise levels tend to be high. To overcome the high noise levels, it is generally necessary to use narrow band filtering, error-detecting and error-correcting codes, and relatively high signal power levels at low bit rates.
The aforementioned problems arise in two areas. The first concerns transmitting information from the central source in the direction of energy flow to the individual customer premises. This transmission of information in the direction of energy flow is referred to as "outbound" signaling. Functions such as automatic meter reading and various alarm systems, however, require that information pass not only from a single source to the end user, but also from the end user back to the central station. This transmission of information in the direction opposite to that of the energy flow is referred to as "inbound" signaling
In the system described in the aforementioned patents, each binary digit (a binary "1" or a binary "0") is made up of four current pulse modulations located at preselected zero crossings of the electrical distribution network voltage waveform. These current pulses are located within eight zero crossings (four complete cycles) of the waveform and the current pulse patterns for "1s" and "0s" are complementary.
By using different pulse patterns to define binary "1s" and "0s," it is possible to define a number of separate, non-interfering channels over which information can be transmitted in each eight half-cycle segment of the waveform. Such channel pulse patterns are explained in U.S. Pat. No. 4,963,853, the disclosure of which is incorporated herein by reference.
Presently available communications systems of this type are generally satisfactory, but they could be improved. For example, noise on the electricity distribution network can significantly reduce the performance of the communications system. The signal wave itself can change its wave shape and frequency spectrum, depending upon conditions on the network. When a message is received which is in error, it is not always possible to identify which bit or bits of the message are incorrect. Moreover, present systems do not generally provide equivalent performance on all non-interfering channels. And these systems suffer reduced performance at higher 60 Hz background currents.